The invention relates to a stepless variable speed drive.
Conventional power transmission devices of the type in question comprise a bevel friction disk, constituting the driving member, and a driven ring; the two members engage in dry contact, the driven ring being urged against the disk by a spring, and are arranged in such a way that the speed of the driven member is infinitely variable in relation to the driving member.
The axis of the driving friction disk is canted in relation to that of the driven ring through an angle substantially complementary to the cone angle of its bevel, in such a way that the surface of the driven ring lies tangential to the surface of the disk at the point where contact is made between them --i.e., in such a way that mating contact covers the maximum area obtainable.
It will be clear enough that frictional contact between the driving and driven members needs to be as great as possible if dependable and long-lasting operation is to be ensured, and accordingly, the environment by which the friction disk and driven ring are accommodated must be clean and dry, in order to ensure a high coefficient of friction.
In conventional single-disk embodiments, the disk and the driven ring are keyed respectively to the driving and driven shafts of the unit, and housed in a fixed case with an adjustable cover; the driving and driven shafts pass in a fluid-tight fit through the adjustable cover and the fixed case, and are supported by them respectively.
The cover generally affords a mounting for the motor by which the friction disk is driven, and is capable of shifting in relation to the fixed case to the end of varying the distance between the axis of rotation of the disk and the area of contact between disk and wheel commensurately with the extent of the shift induced. Given steady input speed, lengthening or shortening this distance produces a corresponding variation in tangential velocity at the area of contact, and as the distance between the area of contact and the axis of the driven shaft remains constant, one obtains a variation in the speed of the driven shaft.
Advantageously, the cover will generally be fitted to the fixed case by way of straight guides in order to obtain an accurate adjustment of the speed of the driven shaft; however, an accompanying disadvantage is that gaps remain between the cover and the case.
The existence of these gaps dictates the ned for seals that are somewhat complex in embodiment, given the type of fit and the variation in the size of the gaps; at all events, the seals in question are not fully able to ensure the exclusion of dust, moisture, oils and other agents that have the effect of lowering the coefficient of friction. Moreover, this loss of friction occasioned by infiltration of foreign matter, or by excessive build-up of moisture internally of the case, is compounded further by the fact that penetration of such agents into the case and between the disk and wheel occurs sporadically, rather than continuously; as a result, transmissible power and torque can vary unexpectedly, and the rated performance of the unit becomes erratic.
One sealing arrangement conventionally adopted consists in the use of a pair of rigid protective elements located between the friction disk and the cover and breasted together in frontal contact, one fastened to the fixed case, the other to the movable cover; a seal of felt or other similar material is attached to one of the two rigid elements, say to the movable element, the covers and entire are of the surface offered to the remaining element.
The movable protective element needs to exhibit a larger surface area on either side of the driving shaft than that of the gap in the fixed element against which it slides, in order to ensure that no gaps remain between the two elements.
In another arrangement, seals are located between the guides of the cover, though in this instance too one has a sealing action between relatively mobile straight surfaces that is insufficiently tight and difficult to embody, as those skilled in the art will be aware.
The result is that stepless variable speed drives of the type in question require regular servicing, and more exactly, the seals must be inspected and/or replaced regularly in order to ensure that abnormal or excessive wear will not further jeopardize what is, in any event, a less-than-perfect sealing action.
Accordingly, the object of the invention is one of embodying a stepless variable speed drive of the single friction disk type, in which fully effective sealing action is ensured against any agent tending to lower the coefficient of friction.
A further object of the present invention is to provide a stepless variable speed drive that affords long-term dependability and is capable of responding stably to its rated performance characteristics.